12 ANIMALS OF THE PAST 
and hard, would probably last for years un- 
der favorable circumstances. ‘Throughout the 
marls and clays of the Bad Lands (of South 
Dakota) there is a large amount of potash. 
This is dissolved by water, and then acts upon 
quartz, carrying it away in solution. ‘This 
would find its way by infiltration into the in- 
terior of the nut. At the same time with this 
process, carrying lime carbonate in solution 
was going on, so that doubtless the stone ker- 
nels, consisting of pretty nearly equal parts of 
lime and silica, were deposited within the nuts. 
These kernels, of course, became hard and 
flinty in time, and capable of resisting almost 
any amount of weathering. Not so the or- 
ganic shell; this eventually would decay away, 
and so leave the filling or kernel of chee 
and lime.* 
“Fossil leaves” are nothing but fine casts, 
made in natural moulds, and all have seen 
the first stages in their formation as they 
* Right here is the weak spot in Professor Barbour's ex- 
planation, and an illustration of our lack of knowledge. For 
it is difficult to see why the more enduring husk should not have 
become mineralized equally mith the cavity nithin. 
